Most of MPEG decoders usually comprise an external memory of 16,8 Mbits (1 Mbit=2.sup.20 bits), or even more when they have to support decoding at main level/main profile. It is known for instance that the incoming bitstream has to be stored in an input buffer (VBV-buffer), the theoretical capacity of which is equal to 1,835 Mbit. However, since a practical decoder cannot decode instantaneously, some additional buffer is required, which increases the input buffer to a capacity of about 2,5 Mbits. Moreover, in order to support a bidirectional motion compensation, the decoder must include two reference frames. The storage capacity for one reference frame is 5 Mbits for 50 Hz systems and 4,1 Mbits for 60 Hz systems. To enable the decoding of both standards, 10 Mbits have therefore to be provided in the decoder for the storage of both reference frames. It must also be recalled that the reconstruction of MPEG video signals after decoding occurs in a macroblock order whereas the information to be displayed is required in an interlaced scanning line format. As the macroblocks are frame coded, the information of both fields has to be delivered during decoding, and the conversion from the frame coded macroblock format to the interlaced scanning line format requires a conversion memory, generally of 5 Mbits or 4,1 Mbits according to the system (50 Hz or 60 Hz). The total memory requirement then becomes equal to (2,5+(3.times.4,1))=14,8 Mbits for a 60 Hz decoder and to (2,5+(3.times.5))=17,5 Mbits for a 50 Hz decoder. For this last one, the practical 16 Mbits limit is therefore exceeded.
Moreover, the integrated circuit technology that is to-day available allows to implement other functionalities than video decoding, for example combinations with audio decoding and bidimensional graphics, and next generation MPEG decoders will probably include demultiplexing functions and improved processors. All these additional functions are memory demanding and their performance is determined by the amount of available memory capacity.
In order to solve this problem, it has been added extra memory (of 4 Mbits) to the MPEG decoder, but such a solution is provisional and will be probably obsolete in a near future. More expensive 16 Mbits memories will then be added.
Another proposed solution, described in the document EP 0618722, has consisted in reducing the memory capacity for display conversion. By using the display memory in a macroblock line fashion rather than on field basis, the capacity of the display memory has been reduced from 5 Mbits to about 3,5 Mbits. The memory requirement for 50 Hz MPEG decoders then falls below the practical limit of 16 Mbits, but due to the so-called 3:2 pull down mode the concerned principle cannot be simply applied in 60 Hz systems.
A further reduction in memory capacity, described in the documents EP 0714208 and EP 0716395, has been achieved by doubling the decoding speed in the MPEG decoder. The field lines required for display can then be directly delivered at the appropriate speed, the non required lines from the opposite field being discarded at the end of the decoding process. With this solution, the memory requirement for 50 Hz decoding is about 13 Mbits, and can also be applied in 60 Hz decoders (about 11,1 Mbits). It is however likely that in a near future the memory requirement for MPEG decoders will strongly increase and that the 16 Mbits frontier will be exceeded even for 60 Hz systems.